Due to the increasing popularity of terminals or Mobile Stations (MSs) such as smart phones, the average amount of data consumed by mobile users has exponentially increased, and the users' demands for higher data rates have also constantly increased.
Generally, a method for providing a high data rate in a mobile communication system may be divided into a method of using a wider frequency band to provide communication, and a method of increasing the frequency use efficiency. It is very difficult to provide a higher average data rate with the latter method, because the communication technologies of the current generation already support the frequency use efficiency close to its theoretical limit, making it difficult to further increase the frequency use efficiency by improving the technologies.
Therefore, a more feasible way to increase the data rate is to provide data services over a wider frequency band. In this case, available frequency bands need to be considered. In the current frequency distribution policy, available broadband communication bands of 1 GHz or more are limited, and the actually available frequency bands include only the millimeter wave (mmW) bands of 30 GHz or more. In these high frequency bands, unlike in the 2 GHz band used by the certain cellular systems, signals are significantly attenuated depending on the distance. Due to the signal attenuation, in the case of a Base Station (BS) that uses the same power as that of certain cellular systems, its service coverage is significantly decreased. In order to solve these and other problems and disadvantages, the beamforming technique is widely used to increase the transmission/reception efficiency of an antenna by concentrating transmit/receive power in a narrow space.
FIG. 1 illustrates a mobile communication system including a MS and a BS that provides beamforming using array antennas.
Referring to FIG. 1, a BS 110 transmits data in each of cells (or sectors) 101, 103 and 105 using a plurality of array antennas Array0 and Array1 by switching the direction of a Downlink (DL) Transmit (Tx) beam 111. An MS 130 also receives the data by switching the direction of a Receive (Rx) beam 131.
In the mobile communication system that performs communication using the beamforming technique, the BS 110 and the MS 130 provide data services by selecting the direction of a Tx beam and the direction of an Rx beam, which show the optimal channel environment, from among a variety of directions of the Tx beam and the Rx beam. The beamforming technique may be equally applied not only to a DL channel carrying data from the BS 110 to the MS 130, but also to an Uplink (UL) channel carrying data from the MS 130 to the BS 110.
In the beamforming technique, if it is assumed that the number of directions of a Tx beam, in which the BS 110 can transmit data, is N, and the number of directions of an Rx beam, in which the MS 130 can receive data, is M, the simplest way to select the optimal DL Tx/Rx direction is that the BS 110 transmits a predetermined signal in each of N available Tx beam directions at least M times, and the MS 130 receives each of N Tx beams using M Rx beams. In this method, the BS 110 needs to transmit a specific reference signal at least N×M times, and the MS 130 receives the reference signal N×M times and measures signal strength of the received reference signal. The MS 130 determines, as the optimal Tx/Rx beam direction, the direction that shows the highest measured signal strength among the N×M measured signal strengths.
As such, the process of transmitting a signal in all possible Tx directions by the BS 110 at least once is called a beam sweeping process, and the process of selecting an optimal Tx/Rx beam direction by the MS 130 is called a beam selection process. This optimal DL Tx/Rx beam selection process may be equally applied even to a UL Tx/Rx process of transmitting data from the MS 130 to the BS 110.
In the common cellular system, a BS transmits a DL reference signal using specific wireless resources reserved for a Sync Channel (SCH) or a reference signal. This DL reference signal is repeatedly transmitted more than once using sufficient transmit power so that all MSs existing in the coverage of the BS receive the DL reference signal. In the mobile communication system that performs communication using the beamforming technique like in FIG. 1, in order to transmit a DL reference signal throughout its coverage, the BS needs to transmit the DL reference signal in all possible Tx directions more than once using the above-described beam sweeping method. The number of transmissions, which is required to transmit a DL reference signal by beam sweeping, is proportional to the number of Tx beams existing in the coverage of the BS 110.
FIG. 2 illustrates a beam width, an elevation angle, and an azimuth in a mobile communication system using beamforming.
It will be assumed in FIG. 2 that a BS 210 is installed in a location having a height 201 of, for example, a building from the ground, and has a predetermined beam width 205. The beam width 205 of the BS 210 can be defined for each of the elevation angle and the azimuth. Generally, the elevation angle refers to an angle (for example, an angle between an antenna and the ground) at which an antenna for transmitting and receiving radio waves sees the satellite. In the example of FIG. 2, since an antenna of the BS 210 looks down at the ground, its elevation angle 203 is construed as an angle between a Tx beam and the vertical surface of the building on which the BS 210 is installed. Although not illustrated in FIG. 2, the azimuth can be construed as an angle of the horizontal direction in which the Tx beam is propagated.
FIG. 3 illustrates an example of a range that a Tx beam reaches depending on the elevation angle in a mobile communication system using beamforming.
It will be assumed in FIG. 3 that a BS 310 is installed on, for example, a building as described in FIG. 2, and has an installation height of, for example, 35 m and the coverage of a radius of about 200 m.
As illustrated in FIG. 3, in the absence of obstacles, a Tx beam transmitted by the BS 310 is transmitted up to a distance of 20 m within the coverage of the BS 310 if its elevation angle is, for example, 25° (see 301); the Tx beam is transmitted up to a distance of 42 m if its elevation angle is 50° (see 303); the Tx beam is transmitted up to a distance of 96 m if its elevation angle is 65° (see 305); and the Tx beam is transmitted up to a distance of 198 m if its elevation angle is 75° (see 307). It can be understood from the example of FIG. 3 that the Tx beam transmitted by the BS 310 reaches up to the farther area as its elevation angle is larger, and as the Tx beam is transmitted farther from the BS 310, its reach is longer and it can be received in the wider area.
FIG. 4 illustrates the number of Tx beams that can be used by a BS depending on the elevation angle and the azimuth.
Specifically, FIG. 4 illustrates the number of Tx beams that can be transmitted by a BS 410, under the assumption that the BS 410 is installed on, for example, a building like in FIG. 2, and the BS 410 is installed at the height of, for example, 35 m, and transmits a Tx beam having a beam width of 5° with respect to each of the elevation angle and the azimuth in one sector having an angle of 30° and coverage of 200 m.
In the example of FIG. 4, since the number of Tx beams that can be transmitted by the BS 410 is a product of 16 elevation-angle Tx directions in units of 5° and 6 azimuth Tx directions in units of 5° for each elevation-angle Tx direction, and is 96 in total, the total number of possible Tx directions of the Tx beams is 96.
Although a Tx beam transmitted by a BS is spread in the form of a sector (or fan) as illustrated in FIG. 3, for the convenience purposes, it is assumed in the example of FIG. 4 that each Tx beam reaches the ground in the form of a rectangle. In FIG. 4, the rectangles mean 96 areas where a Tx beam having specific azimuth and elevation angle has reached the ground. As described in conjunction with FIG. 3, the mean 96 Tx beams are transmitted up to the farther region as their elevation angle is greater, and as the Tx beams are transmitted farther from the BS, their reach is longer and they are received in the wider area. A ratio written in each rectangle represents the ratio of a reception (Rx) area of the Tx beam transmitted to the location of the rectangle, to a total of 96 areas, in terms of the size. It can be understood that as illustrated in FIG. 4, even for a Tx beam having the same beam width, a Tx beam that is transmitted to the region close to the boundary area of the BS is received in the much wider area depending on the elevation angle and azimuth, compared to a Tx beam that is transmitted to the region close to the central area of the BS. A simulation showed that in the example of FIG. 4, where the BS has a height of 35 m and the coverage of 200 m, there is a size difference of a maximum of 480 times between Rx areas of a Tx beam.
As illustrated in the example of FIG. 4, if a Tx beam having the elevation angle and azimuth of a narrow beam width is used, a plurality of possible Tx beams and Rx areas exist in the coverage of the BS. In the example of FIG. 4, the BS needs to repeatedly transmit a DL reference signal at least 96 times, if it transmits a DL reference signal more than once in all possible Tx directions by beam sweeping. Since the number of transmissions required to transmit a DL reference signal by beam sweeping is proportional to the number of Tx beams available in the coverage of the BS, the simplest way to reduce the Tx overhead of the DL reference signal in the BS of FIG. 4 is to support the full coverage of the BS with a lesser number of Tx beams. To this end, each Tx beam needs to have a wider beam width. For example, in order to support a sector of 60° with two Tx beams, each Tx beam needs to have a beam width of about 30°.
Generally, however, as a Tx beam has a wider beam width, its beamforming effects are lower in proportion thereto. In other words, as a beam width is narrower, the beamforming effects are higher. If a beam width is reduced to increase the beamforming effects, the number of Tx beams needed to support one BS area increases according to the beam width reduction, causing an increase in the overhead needed to transmit a DL reference signal. As such, the beamforming effects and the transmission overhead have a trade-off relationship with each other.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable with regard to the present disclosure.